Sleep Deprivation Affects Somatosensory Cortex Excitability as Tested Through Median Nerve Stimulation

Background: Changes of cortical excitability after sleep deprivation (SD) in humans have been investigated mostly in motor cortex, while there is little empirical evidence concerning somatosensory cortex, and its plastic changes across SD. Objective: To assess excitability of primary somatosensory cortex (S1) and EEG voltage topographical characteristics associated with somatosensory evoked potentials (SEPs) during SD. Methods: Across 41 h of SD, 16 healthy subjects participated in 4 experimental sessions (11.00 a. m. and 11.00 p. m. of the 1st and 2nd day) with: a) subjective sleepiness ratings; b) EEG recordings; c) SEPs recordings; d) behavioral vigilance responses. Results: A clear enhancement of cortical excitability after SD was indexed by: (a) an amplitude increase of different SEPs component in S1; (b) higher voltage in occipital (around 35-43 ms) and fronto-central areas (around 47-62 ms). Circadian fluctuations did not affect cortical excitability. Voltage changes in S1 were strongly related with post-SD fluctuations of subjective and behavioral sleepiness. Conclusions: Sleep may have a role in keeping cortical excitability at optimal (namely below potentially dangerous) levels for the human brain, rebalancing progressive changes in cortical responsiveness to incoming inputs occurred during time spent awake. On the other hand, higher level of cortical responsiveness after sleep loss may be one of the mechanisms accounting for post-SD alterations in vigilance and behavior.